The invention relates to a distance measuring apparatus, and more particularly, to a distance measuring apparatus which projects a light beam toward an object to be determined and receives reflected light therefrom to determine the distance to the object.
An automatic focussing mechanism which may be used in a compact camera or the like generally comprises a distance measuring apparatus of the double image coincidence type which is based on the triangulation technique, and which is commonly referred to as a distance measuring apparatus of the so-called Viditronics type. A distance masuring apparatus of this type comprises a pair of arrays, each of which includes a plurality of photoelectric transducer elements and on which the image of an object being photographed is projected in accordance with the triangulation technique. A distance measurement is made in this arrangement, on the basis of determining the coincidence of double images in terms of a minimum sum of differential outputs from pairs of transducer elements, each pair being formed by two of the transducer elements from the two arrays which are located in a corresponding manner relative to each other.
However, a distance measuring apparatus of the Viditronics type suffers from the following disadvantages:
(1) An accurate determination of distance is not assured unless photoelectric conversion characteristics of both arrays are identical with each other as are the optical characteristics of incident paths of the light from an object being photographed which impinges upon both arrays, since the conversion outputs are then not balanced. However, it is very difficult to provide identical characteristics for both arrays or the related optics, resulting in an instability in the accuracy of the measurement achieved.
(2) The use of the triangulation technique requires a scan over distances from infinity to the closest point, which is usually performed by a rotation of a movable mirror. However, the resulting mechanism becomes complex, rendering it difficult to assure an increased reliability.
(3) The movable mirror is driven for rotation by the power charged during a film winding operation. Thus, it is necessary to provide a mechanism for interlocked movement with the film winding mechanism, adding to the complexity of the arrangement.
(4) The scan process comprises initially rotating the movable mirror from a start position corresponding to infinity to a terminal position corresponding to the closest point, during which rotation a point is detected where the sum of differential outputs from the transducer elements is at its minimum, followed by a reversed motion of the mirror to allow an interlocked movement of taking lens optics to the point where the minimum sum is obtained. Such a two step process is commonly employed, resulting in a time loss which is wasted from the distance measurement to the photographing operation. If an object being photographed happens to move during such measuring interval, the camera cannot be focussed upon the intended object. Accordingly, this approach does not lend itself to an advanced camera such as single lens reflex camera.
Distance measuring apparatus other than the Viditronics type is also known, which is again based upon the triangulation technique. In the triangulation technique, a distance to an object being photographed is determined from a base line across a pair of spaced points on the camera, which is very small as compared with the distance to the object. In other words, the determination is based on the detection of a change in a minimal angle formed when the object is subtended by the pair of points on the camera. It is difficult to provide an accurate measurement output corresponding to such a change in the minimal angle. Accordingly, the distance measuring apparatus involves complex signal detection and processing, requiring a complex arrangement of an increased precision which results in an expensive arrangement.